Composite materials of resin and reinforcing fiber are used for various purposes because of their excellent properties such as light weight, rigidity and impact resistance. Especially, since carbon-fiber-reinforced composite materials are lightweight with highly strong and rigid features, they are used for a wide range of purposes, for example, sports/leisure applications such as fishing rods and golf shafts, industrial applications such as automobiles and aircraft, and so on. In recent years, since carbon fibers have magnetic wave shielding properties in addition to mechanical characteristics, carbon-fiber-reinforced composite materials are also used as housing for electronic/electrical devices such as laptop computers.
Flame retardance is one of the various properties expected when fiber-reinforced composite materials are used. For example, when used as housing for electronic/electrical devices, flame retardance is required in consideration of the potential for fire caused by heat generated in a device.
For setting fiber-reinforced composite material to be flame retardant, methods for adding brominated epoxy resin to the matrix resin have been widely employed. However, considering the toll on humans and the environment from toxic substances generated when resin compositions containing halogen are burnt, methods such as adding red phosphorus or phosphate compounds to epoxy resin (for example, patent publication 1), and adding phosphazene compounds to epoxy resin, for example, have been mainly employed recently as a way for obtaining flame retardance without using brominated epoxy resin. However, those methods have problems: 1) mechanical strength decreases when the amount of added substances increases; 2) storage stability is low; 3) red phosphorus, phosphate and phosphazene compounds seep out (bleed out) gradually over a long period; 4) hydrolysis of red phosphorus or phosphate compounds occurs easily; and 5) when the amount of added phosphate compounds or phosphazene compounds is large, the glass transition temperature (Tg) of cured material of the matrix resin is lowered, and the heat resistance of the cured material decreases. Accordingly, using the above methods, flame-retardant properties can be added to fiber-reinforced composite materials only to a certain degree, and their stability is low. Especially, considering the problem described in 4) above, it is difficult to apply such a method of using red phosphorus or phosphate compounds in the field of printed wiring boards or electronic materials where high insulation and water resistance are required.
To form a fiber-reinforced composite material, a method of using prepreg, which is an intermediate material formed by impregnating reinforcing fiber with thermosetting resin, is usually employed. A molded article made from fiber-reinforced composite material is obtained, for example, after the prepreg is cut and molded into a desired shape, and then is thermally cured in a die.
Epoxy-resin-based prepreg containing epoxy resin as a thermosetting resin is widely used. Since epoxy resin is hard to cure by itself, it is usually formulated with a curing agent or curing accelerator. However, even when formulated with the curing agent or curing accelerator, since the molding time is long (duration until the curing process is completed) for epoxy-resin-based prepreg, it is difficult to apply the prepreg to members requiring mass production such as automobile parts.
On the other hand, performing press molding at elevated temperatures and pressures is known as a high-cycle molding method with high productivity and is widely employed in automobile applications. In such a press molding, methods such as raising the molding temperature to reduce the curing time are employed to further enhance productivity. However, if the molding temperature is raised to be at least approximately 20° C. higher than the glass transition temperature (Tg) of the material to be cured, such a procedure results in a soft cured article. When the cured article is taken out from the molding die under such conditions, deformation or the like may cause problems. Therefore, it is necessary to cool the molding die before taking out the cured article from the die, which is not preferable considering the high-cycle aspect.
As epoxy-resin compositions capable of curing in a shorter period of time at lower temperatures, patent publication 2 discloses an epoxy-resin composition made of epoxy resin, an amine compound containing at least one sulfur atom in the molecule and/or a reacted product of epoxy resin and an amine compound containing at least one sulfur atom in the molecule, a urea compound, and dicyandiamide, along with prepreg produced made of such an epoxy-resin composition. Also disclosed in patent publication 2 is a method for forming fiber-reinforced composite material by press-molding the prepreg. As for urea compounds, 3-(3,4-dichlorophenyl)-1,1-dimethylurea or phenyl dimethyl urea is used.
However, more improvements should be made regarding the curability of the epoxy-resin composition above. In addition, the Tg of the obtained cured product is low, and its heat resistance needs to be improved.